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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Tuck82er on March 11, 2013, 07:14:16 PM

Title: Resonance forms of 1,3-cyclohexadiene
Post by: Tuck82er on March 11, 2013, 07:14:16 PM
Was wondering why 1,3-cyclohexadiene only has three total resonance forms and not four. I have:

1,3-cyclohexadiene :resonance: 1-anion, 2-cation, 3,4-ene  :resonance: 1-anion, 2,3-ene, 4-cation  :resonance: 1,2-ene, 3-anion, 4-cation

Sorry the notation is a little odd. Wasn't sure how to insert actual structures.

Thanks
Title: Re: Resonance forms of 1,3-cyclohexadiene
Post by: camptzak on March 12, 2013, 02:23:44 AM
something has to attack the conjugated system to produce actual resonance structures, otherwise the four electrons in the conjugated system are just delocalized over the 4 carbons in the system.

if the system was attacked by Cl2 there would be two resonance structures.
Title: Re: Resonance forms of 1,3-cyclohexadiene
Post by: Tuck82er on March 12, 2013, 04:20:18 PM
So are you saying that I wouldn't see any resonance forms at all? I thought molecules could take on resonance forms without the addition of some molecule.
Title: Re: Resonance forms of 1,3-cyclohexadiene
Post by: Dan on March 12, 2013, 04:58:04 PM
Quote from: Tuck82er on March 11, 2013, 07:14:16 PM
Was wondering why 1,3-cyclohexadiene only has three total resonance forms and not four. I have:

1,3-cyclohexadiene :resonance: 1-anion, 2-cation, 3,4-ene  :resonance: 1-anion, 2,3-ene, 4-cation  :resonance: 1,2-ene, 3-anion, 4-cation


Highlighted resonance structures are identical, so there are 3.
Title: Re: Resonance forms of 1,3-cyclohexadiene
Post by: sjb on March 12, 2013, 05:33:23 PM
Quote from: Dan on March 12, 2013, 04:58:04 PM
Quote from: Tuck82er on March 11, 2013, 07:14:16 PM
Was wondering why 1,3-cyclohexadiene only has three total resonance forms and not four. I have:

1,3-cyclohexadiene :resonance: 1-anion, 2-cation, 3,4-ene  :resonance: 1-anion, 2,3-ene, 4-cation  :resonance: 1,2-ene, 3-anion, 4-cation


Highlighted resonance structures are identical, so there are 3.

Are they? One has the cation next to the double bond, the other the anion.
Title: Re: Resonance forms of 1,3-cyclohexadiene
Post by: Dan on March 12, 2013, 08:05:35 PM
My mistake, sorry, I didn't read it properly.
Title: Re: Resonance forms of 1,3-cyclohexadiene
Post by: orgopete on March 13, 2013, 01:12:30 AM
I am going to offer up a guess on this one. Resonance theory is an attempt to explain why there is an energy stabilization in certain instances in which electrons interact. If we were to ask whether localization of a double bond were better with the cation adjacent to the alkene (and thus able to be stabilized) versus the anion adjacent to the alkene, I think there is greater stabilization of a cation. I suggest this even though one could write resonance structures for either one. I think if we were to consider these as two separate entities, a resonance stabilized anion adjacent to cation would be less stable than a resonance stabilized cation adjacent to a anion. (Perhaps someone could calculate the energy differences to determine whether my suggest were correct or not.)